What should you know about delayed fainting after giving blood?

A donation can leave you briefly more vulnerable to light-headedness, especially if you stand quickly, exercise, become overheated, skip food or have not replaced fluids. A reflex faint can slow the heart and widen blood vessels for a short time, reducing blood flow to the brain. The episode may happen after you have left the donation centre rather than immediately.

Anaemia, an irregular heart rhythm such as atrial fibrillation, dehydration or hypoglycaemia can also contribute to fainting or make recovery harder. A fall can cause injury, so the event deserves attention even if you feel normal afterward.

At the first warning of warmth, nausea, sweating, blurred vision or weakness, lie flat and raise your legs if possible. Do not drive or walk alone while dizzy. Once fully alert and able to swallow, take fluids and a snack, then rise slowly. Tell the blood donation service what happened and follow its advice before donating again.

If you struck your head, have ongoing symptoms, or fainted without warning, arrange medical assessment. Keep a note of the timing, food and fluids taken, position, injuries and any medicines.

Call emergency services for chest pain, trouble breathing, a severe or persistent headache, weakness on one side, confusion, seizure, a serious injury, heavy bleeding, or fainting during exertion. Urgent assessment is also needed if you do not wake promptly, faint repeatedly, have a racing or irregular heartbeat, or cannot keep fluids down. Do not assume a delayed episode is only a donation reaction.

Could the fainting relate to blood pressure, anaemia, dehydration, low blood sugar or heart rhythm? Do I need an examination or tests before donating again? Which symptoms should make me seek emergency help? How should I prepare with food, fluids and rest?